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| include | ||
| ld | ||
| src | ||
| make-booster.c | ||
| Makefile | ||
| README.md | ||
Booster: the Foboot Updater
Foboot cannot update itself. It resides at offset 0, and writes to offset
0x40000. Any errors in writing would result in an unbootable device.
Booster is used to guide the installation of Foboot. Much like a booster
rocket helps guide a payload into orbit, Booster guides the installation of
Foboot, and is then discarded.
Usage
First, compile fobooster. Then, append the application header, and finally append the application itself. This can be accomplished with make-booster.
cd toboot/
make
cd ../booster/
make
gcc make-booster.c -o make-booster
./make-booster [flash-id] ../toboot/toboot.bin toboot-booster.bin
The following flash-id values are known:
EVT: 0xef177018 PVT: 0xc2152815 Hacker: 0x1f148601
The resulting foboot-booster.bin can be flashed with Foboot itself as
an ordinary program.
Design
The goal of Booster is to load a new bitstream onto Fomu. Because this bitstream contains code + a CPU, Booster will actually run from within the context of this new CPU.
THE NEW IMAGE IS ASSUMED TO BE 0x19700 (104192) BYTES LONG. The new image is padded by appending 0-bytes to it in order to get it to the correct length.
The SPI flash image looks something like this:
| Hex Address | Decimal Address | Purpose |
|---|---|---|
| 0x000000 | 0 | Instructs the FPGA where to find SB_WARMBOOT images S0-S3 and for cold boot |
| 0x0000a0 | 160 | Normal offset for Foboot |
| 0x040000 | 262144 | New version of Foboot, including new CPU Core |
| 0x05a000 | 368640 | Start of Booster program |
| 0x05a004 | 368644 | Booster signature |
| 0x05a008 | 368648 | Booster length (not including bitstream / new image) |
| 0x05a00c | 368652 | Booster checksum |
| 0x05a010 | 368656 | Actual length of new image |
| 0x05a014 | 368660 | Length of xxHash region |
| 0x05a018 | 368664 | xxHash seed |
| 0x05a01c | 368668 | SPI ID of the target device |
| 0x1FFFFF | 20097151 | End of flash |
The "Booster Signature" is 0x4260fa37 -- if it contains this value, then
this might be a Booster program.
The "Booster Checksum" is the result of summing together [Booster length]
8-bit bytes, starting at offset 0x05a010. If this value is equal to
Booster checksum, then Foboot will simply transfer control over to address
0x05a000.
Booster execution flow
The first thing Booster does is validate the new image. It does this by
performing an xxHash of the new image, from address 0x040000. If the image
does not match the hash, it will erase itself and show an error.
Next, Booster will validate that the start of the target image has a valid SB_WARMBOOT block. This specialized check ensures we don't accidentally write an invalid image.
If the address matches, it checks the SPI ID. If the ID does not match, then it will erase itself and show an error.
If the ID does match, the update will begin.
The first step of the update is to erase block 0 and reprogram it with a temporary block that will point execution to address 0x400a0. This offset means that we can switch from the updater to Foboot by rewriting the execution address from 0x4000a0 to 0x0000a0.
From this point forward, Booster is effectively the only program on flash.
The